The invention relates to a coating which is applied to a substrate, a solar cell comprising the coating according to the invention, and a method for applying the coating to a substrate.
It is known that, for solar cells, the upper electrode needs to meet at least the following requirements in order to effect a good efficiency:
1. the electrode needs to have a high transparency for the incident light;
2. the electrode needs to effect a good conduction of the current generated in the active layer; and
3. the surface morphology of the electrode needs to be such that the incident light is captured in the solar cell.
Known materials which can meet these requirements are the so-called transparent conducting oxides such as tin oxide, zinc oxide or indium tin oxide. These are semiconductors with such a bandgap that light in the visual spectrum is transmitted. In order to effect a right conduction, and thus meet the second requirement, a limited number of free electrons are created in this material by addition of dopant and/or by creating oxygen deficiencies.
In order meet the third requirement, the upper electrode needs to have an optimal surface morphology so that the incident light is refracted and the distance covered through the underlying active layer increases such that the efficiency of the solar cell is considerably improved.
Such an application is for instance known from Japanese patent specification 05 067797 in which an upper electrode is described which comprises tin oxide of which the dominant crystalline orientation is the (200) orientation, and with which an increased efficiency of a solar cell is effected.
However, recent research has shown that, with the upper electrodes used in practice, which are also described in the Japanese patent specification, the requirement of a good electrical conductivity of the upper electrical electrode is at odds with the requirement that the electrode also needs to have an optimal surface morphology. It can be concluded from this that the efficiency of the known solar cells is by no means optimal yet.
It has now surprisingly been found that an upper electrode for a solar cell can be developed which has both a good electrical conductivity and has an optimal surface morphology, when the electrode consists of a coating which comprises at least two films which both comprises a transparent conducting oxide and an electron donor, and the contents of electron donor differ in the two films.
It has further surprisingly been found that, in contrast to what is known from Japanese patent specification 05 067797, a (200) dominant orientation is no condition for an optimal capture of the light in the solar cell and that a capture which is at least as good can be obtained with a dominant (211) and (110) orientation.
Although the (200) orientation and (110) orientation are dominant in a normal process control—as described in the aforementioned Japanese patent specification—it has surprisingly been found that, under specific process conditions, such as for instance an elevated content of electron donors, the (211) orientation becomes dominant. It has further surprisingly been found that a next film which is then applied to this film ‘inherits’ this dominant orientation, even if the film would normally not yield a dominant (211) orientation.